The invention relates to an blade wheel for centrifugal pumps, comprising at least one row of blades positioned successively in the peripheral direction of the blade wheel, the blade ends close to the periphery being at an oblique angle α with respect to the direction of movement of the periphery of the blade wheel. Centrifugal pumps are used widely for the transfer of liquid materials and mixtures, the pump¬ ing effect being created by the rotation of the blade wheel in the fixed casing of the pump. At the same time, the blade wheel transfers the material to be pumped with it, so that the material is subjected to a centrifugal effect by the rotation movement, and when the material reaches the discharge opening, it is discharged into it under the influence of the centrifugal force and the additional influence caused by the obliqueness of the blades. Such pumps are dis¬ closed, e.g., in DE Offenlegungsschrift 2525316 and FI Patent 53747.

A problem with pumps known from the prior art is the pulse-like pressure variation occurring in their discharge conduit, which is disadvantageous under certain operating conditions. In particular, when the pump is used as a feed pump for fibre suspension in the conduit system associated with the head box of a paper machine or the like, the pressure variations cause wave-like variation in the forming paper or cardboard web, thus deteriorating the paper quality. In an attempt to decrease the pulse, the ends of the blades of the blade wheel of the pump have been made oblique in the peripheral direction of the blade wheel, which, however, has not eliminated

the pulse disturbances.

The object of the present invention is to provide an blade wheel by means of which the pulse disturbances can be decreased and which is more suit- able for use in feed pumps in connection with the head boxes of paper machines, for instance. The blade wheel of the invention is characterized in that in order to reduce pressure variations caused by the blades in a discharge conduit of the pump, a distance between the blades and the angle are such that the leading edge of the end of the backward blade in the direction of movement of the periphery of the blade wheel is in alignment with the trailing edge of the end of the forward blade in the direction of movement, or ahead of said trailing edge in said direction of movement.

An essential feature of the invention is that the blades of the blade wheel are disposed at such intervals and at such an angle that when the blade wheel rotates at least one blade end is always pass¬ ing by the nose of the spiral casing, whereby the pressure pulse created by the blade end at the nose remains substantially constant all the time, and no appreciable pressure variation occurs in the dis- charge conduit.

The invention will be described in greater detail in the attached drawings, in which

Figure 1 is a schematic axial sectional view of a centrifugal pump provided with an blade wheel of the invention;

Figure 2 illustrates schematically the edge of the blade wheel of the pump of Figure 1, and the po¬ sition of the ends of the blades;

Figure 3 illustrates schematically pressure variation in a discharge conduit of the pump of

Figures 1 and 2;

Figure 4 is a schematic axial view of a two- sided blade wheel accomplished according to the in¬ vention; and Figure 5 illustrates the blade wheel of Figure 4 as seen from the direction of the blade ends.

Figure 1 shows a centrifugal pump comprising a spiral pump casing 1 within which an blade wheel 3 rotating around a shaft 2 and having blades 4a and 4b is disposed. The pump casing 1 comprises a discharge conduit 5 having a nose 6 at its edge on the side of the casing close to the blade wheel. During the rotation of the pump, the material to be pumped, such as a liquid or a suspension, enters in a manner known per se in the direction of the pump shaft 2 at the centre of the blade wheel, and while the blade wheel rotates, the material is passed by the suction effect created by material discharged into the discharge conduit and by the push effect produced by the pump blades towards the outer edge of the blade wheel 3, and further under the influence of the centrifugal force into the discharge conduit 5. The blade wheel 3 comprises vanes 4a and 4b, of which the blades 4a are conventional in length and extend from the periphery of the blade wheel closer to the pump shaft 2 than the blades 4b, which are shorter and serve as a kind of auxiliary blade. The purpose of the shorter auxiliary blades 4b is to maintain the centre of the blade wheel 3 wider so as to facilitate the flow of mass; on the other hand, it is necessary for the in¬ vention that there are blades and blade ends suf¬ ficiently densely along the outer periphery of the blade wheel 3. In other respects, the structure, operation and dimensions of the centrifugal pump are obvious to one skilled in the art and will not be de-

scribed more closely here.

Figure 2 shows schematically a portion of the edge of the blade wheel 3 of Figure 1, the blade ends, and the nose 6 of the discharge opening. In the figure, the ends of the blades 4a and 4b are disposed at an oblique angle α in such a way that one edge of the blade 4a and the opposite edge of the blade 4b, that is, in the figure the lower edge of the blade 4a and the upper edge of the blade 4b, are in alignment or partly overlap, i.e. a distance A between the same edges of the blades is smaller than a distance B be¬ tween the opposite edges of the same blade in the same direction. When the blade wheel 3 then rotates in the direction indicated by the arrow shown in Figure 2, there are always one or more blades 4 posi¬ tioned at the nose 6; preferably in sμch a way that the number of blades remains the same all the time. The pressure pulse created at the nose by the move¬ ment of the blade remains substantially constant even though its location varies to some extent in the direction of the height of the nose, that is, in the axial direction of the blade wheel. This, however, does not substantially affect the pressure pulse or pressure pattern occurring in the discharge conduit 5a.

Figure 3 illustrates schematically the pressure occurring in the discharge conduit 4 of the centri¬ fugal pump of Figures 1 and 2. The pressure pattern is slightly wave-like, as shown by the continuous wavy line P. The figure further shows two wave-like pulses Pa and Pb, which represent the pulse pattern created by successive blades. As the position of the blades is such as shown in Figures 1 and 2, the pressure pulses are, however, summed so that the pressure curve P is achieved, in which variation is

substantially negligible in practice.

Figure 4 illustrates the principle of Figures 1 and 2 when applied to a two-sided blade wheel. In this case, the blades 4 may have full length and be positioned on opposite sides of the central flange of the blade wheel, or they can be intermediate blades similar to those shown in Figures 1 and 2, whereby the full-length blades may be positioned beside each other, or they may alternate in such a way that the auxiliary blade of one blade row is positioned beside the full-length blade of the other blade row.

Figure 5 shows schematically a portion of the edge of the blade wheel of Figure 4 as seen from the end of the blades. In this case, the blades 4a to 4c, etc., are positioned in adjacent rows at an angle α oblique in reverse directions, so that the edges of the blades 4a and 4b facing each other and cor¬ respondingly the edges facing away from each other extend in the same direction with respect to the direction of rotation of the blade wheel 3. The embodiment of Figures 4 and 5, concerning the two- sided blade wheel, is a pump in which the material or mass to be pumped enters the pump on its both sides in the direction of the shaft at the centre of the blade wheel and is then passed on into a common dis¬ charge conduit. As shown in Figure 5, the leading edges of the adjacent blade rows in the direction of movement are on the outer edges of the blade wheel, while the trailing edges in the direction of movement face towards the central flange of the blade wheel 3. Similarly as in Figure 2, the blades 4a and 4b and correspondingly the blades 4b and 4c, etc., are posi¬ tioned so that the backward edge of the blade 4a and the forward edge of the blade 4b in the direction of movement of the blade wheel, i.e. in the direction

indicated by the arrow, are substantially in align¬ ment, or so that the blades partly overlap in the peripheral direction. Correspondingly, the backward edge of the blade 4b and the forward edge of the blade 4c are similarly positioned with respect to each other. Preferably, the blade ends are sub¬ stantially in alignment in this case too, whereby the number of blades or blade portions at the nose 6 is always the same, so that the pressure variation is minimized.

The invention has been described above and in the drawings schematically and by way of example, and it is in no way restricted to this example. In place of blades of different lengths, it is possible to use blades equal in length while the number and density of the blades may be chosen suitably according to the desired evenness of the pressure. Similarly, the curvature and the obliqueness of the blades may vary over the length of the blades, provided that the above-described principle is observed at the ends of the blades.